1. Field of the Invention
This invention relates to polyolefin/polyester composite fibers having a low apparent Young's modulus and a filter element using the same.
2. Description of the Related Art
A process for producing a hollow cylindrical filter element by winding up a web composed of polyolefin composite fibers on a core while heating the web is disclosed in Japanese patent publication No. Sho 56-43139/1981. According to the process, it is possible to obtain a filter element having a desired porosity by controlling the pressure at the time of winding-up, but since the raw material of the element is of polyolefin composite fibers, there has been raised a drawback that the upper limit of the service temperature is as low as 80.degree. to 90.degree. C., and also there has been raised a problem that the filter element swells depending on the liquid passing through the element.
Further, in order to obtain a filter element endurable to its use at high temperatures, there have been an attempt of replacing the above-mentioned polyolefin composite fibers by polyester fibers using a low melting non-crystalline polyester as the sheath component thereof and a high melting crystalline polyester as the core component thereof, but since the low melting polyester is vitreous, there has been raised a problem that the low melting polyester peels off in the form of powder at the steps of spinning and stretching or at the steps of production and use of the filter element. Further, when polyolefin/polyester composite fibers using polyethylene as the sheath component thereof and a crystalline polyester as the core component thereof are used, there is a drawback that since the apparent Young's modulus of the polyester is high, the resulting filter element is bulky and low in density; hence it is not only difficult to catch fine particles, but also the element is low in hardness and soft, and hence not endurable to practical use.